Mechanical analysis of the collaring process of the 11 T dipole magnet

As part of the Large Hadron Collider (LHC) accelerator upgrades foreseen by the high luminosity-LHC project, the CERN 11 T program is aimed at replacing standard LHC Nb-Ti main dipole magnets, operating with a bore field of 8.3 T, with pairs of shorter Nb$_3$Sn dipole magnets with a bore field of 11 T and the same total integrated field, thus providing space for additional collimators in the dispersion suppressor region. At the time of the submission of this paper, six single-aperture and two double-aperture short models have been fabricated and tested. As a result of a degraded quench performance observed in some of the short models, attributed to excessive stress on the Nb$_3$Sn coil mid-planes, a thorough investigation of the room temperature loading procedure, and in particular of the collaring process, has been launched. A 150-mm-long collared coil mockup, instrumented with strain gauges and pressure sensitive films, has been used to study the peak stresses experienced by the brittle and strain sensitive Nb$_3$Sn cables in the different phases of the collaring and as a function of coils' size and collaring force. In this paper, the results of the test campaign are described.